This invention relates generally to an automatic differential locking system for a working vehicle, and more particularly to an automatic differential locking system for automatically limiting differential revolutions of front wheels or rear wheels in response to a selected steering mode.
When a working vehicle travels on an inclined ground or an unleveled ground, slipping causes a difference in revolution between a right wheel and a left wheel impairing straight running of the vehicle. It is therefore conventional practice to equip a working vehicle such as a lawn and garden tractor which needs to run straight in a reliable manner with a differential locking mechanism to forcibly stop differential revolutions of the wheels. However, when the vehicle is steered with the differential locking mechanism working at all times, one of the right and left wheels must slip and leave undesirable scratches on the ground.
In order to eliminate the above disadvantage an automatic differential locking system has been proposed in Japanese patent application published under No. (Tokugansho) 58-246744 (Kubota, Ltd.). This prior system permits the differential to be locked only when a steering angle is below a predetermined value, and releases the lockup when the steering angle exceeds the predetermined value.
On the other hand, there is a known steering mode called parallel steering in which the front and rear wheels movable parallel to one another are steered in the same direction. This steering mode is useful for steering the vehicle clear of an obstacle encountered while running, and for altering a vehicle position relative to a running course or moving the vehicle sideways. All these operations can be carried out without turning the vehicle. Since the parallel steering mode turns all the four wheels in one direction, there occurs no difference in wheel revolution and therefore little chance of slipping. Thus the parallel steering mode is particularly effective on an inclined ground.
According to the system proposed in the above noted Japanese application, the differential lockup is automatically released regardless of the steering mode when an amount of steering exceeds a predetermined value. The proposed system is therefore unable to avail itself of the effective parallel steering mode.